JOP060: Joint VLA-SOHO Observations of Transient Sources in the Solar Corona
Proposers: Robert Willson and Kenneth R. Lang
Department of Physics and Astronomy=20
Tufts University
Medford, MA 02155
(rwillson@pearl.tufts.edu)
PROJECT DESCRIPTION
VLA observations at 2 and 3.5 cm wavelength have shown that the
quiescent emission from active regions originates in compact (size~5")
highly circularly polarized (up to 90%) sources in regions of strong
magnetic fields near sunspots (Willson and Lang 1986). The observed
brightness temperatures of Tb 1-2 x 10**5 K suggests that this emission
occurs in the transition region or low corona. Other VLA observations
have revealed a different class of 2 cm sources which are also compact
and sometimes circularly polarized but which are located away from
sunspots in regions of apparently weak (B 100 G) photospheric fields
(Willson and Lang 1987). These sources are often time variable,
changing on timescales of minutes to hours.
These microwave sources are enigmatic because magnetic fields of
1500-2000 G are required if the observed brightness temperatures
and circular polarization are to be attributed to thermal gyroresonance
radiation or propagation effects (thermal bremsstrahlung).
Alternatively, the 2 cm emission could be due to the gyrosynchrotron
radiation of mildly relativistic electrons accelerated in regions of
changing magnetic flux (Willson and Lang 1986). The observed
variability could then be caused by fluctuations in magnetic field
strength or to variations in electron density. Similiar conclusions
were reached by Gopalswamy et al. (1994) who detected transient
microwave brightenings (2 cm) in association with soft X-ray
brightenings from active regions.
The timescale of the fastest 2 cm variations is similiar to those
of small-scale transient events that have been observed at optical,
ultraviolet or X-wavelengths. Examples of such transient brightenings
include H alpha surges (Schmieder et al. 1993), small-scale UV
explosive events (Dere et al. 1989), high velocity UV jets (Brueckner
and Bartoe, 1983), X-ray and UV bright points and X-ray jets (Hanaoka,
1994). These eruptive or explosive events are believed to be triggered
by the magnetic reconnection of oppositely-directed magnetic fields,
perhaps in response to motions at lower levels in the photosphere. The
process of magnetic reconnection may produce energetic particles that
escape from the reconnection site and produce characteristic
signatures at different heights or locations in the active region.
The high spatial resolution of SUMER, CDS and EIT can be used to
image the intensities and profiles of temperature and
density-sensitive lines, thereby establishing the physical conditions
in which these evolving sources arise. Here we propose to use the VLA
in conjunction with SOHO to produce images of total intensity and
circular polarization at 2 and 3.5 cm that can provide additional
information about the magnetic field strength within the regions of
energy release in the transition region and corona.
The combined VLA/SOHO data will be used to investigate transient
phenomena in the chromosphere and transition region, magnetic
changes in the underlying photosphere and thermal and nonthermal
particle acceleration in the corona. Specific questions which we hope
to answer include:
1. What radiation mechanism can account for the brightness and circular
polarization? observations of the transition region/low corona at 2
and 3.5 cm should be able to determine whether it is thermal or
nonthermal. It is important to note that the UV data alone cannot
determine whether the transient energetic sources produce nonthermal
emission; it is only the microwave data, through measurements of
brightness temperature and circular polarization, that can distinguish
between a thermal or nonthermal radiation mechanism.
2. What is the spatial and temporal distribution of the UV and radio
sources and what does this comparison tell us about their magnetic
structure and particle acceleration?
3. Is the variability due to intermittent heating, continued low-level
flare activity, changing magnetic field topology or any combination of
these? VLA snapshot maps at 2 and 3.5 cm will be able to study source
variations at these wavelengths and to monitor spectral and
polarization variations that give clues to magnetic field changes.
PROPOSED VLA_SOHO OBSERVATIONS OF TRANSIENT EVENTS
We have been given three periods of VLA observing time to observe the
Sun on April 15, 17 and 19 from approximately 1500-1930 UT on each
day. It is also possible that we will have up to three hours of VLA
time on April 10 and 12 - the VLA will let us know within the next
month about these additional observations. The entire array will be
used to switch between 2 and 3.5 cm every few minutes while also
observing continuously at 91 cm The requested time intervals will allow
us to study the temporal evolution of transition region sources while
also allowing investigations of different types of photospheric
magnetic configurations including active regions and areas of "quiet"
Sun. Snapshot maps of total intensity and circular polarization at 2
and 3.5 cm (on time intervals of 1.7 seconds) will be used to study
magnetic and/or temperature changes in the transition region and low
corona that may be produced during transient events that originate in
the same or lower levels. Images made simultaneously at 91 cm may
reveal decimetric Type III-like bursts if energetic electrons are
produced and escape along open magnetic field lines anchored to the
sites of reconnection events.
Here we propose that our VLA observations be part of a Joint
Observing Program (JOP) involving the SOHO CDS, EIT and SUMER. The
objective of these SOHO observations would be to study the transition
region and low-corona counterparts of transient brightenings and
evolving 2 cm and 3.5 cm sources. CDS might be used to make raster
scans of one or two spectral lines (over a 120" x 120" field of view
every 5 - 10 minutes) with peak formation temperatures (T == 1 - 10 x
10**5 K) that are comparable to the expected brightness temperatures of
the evolving 2 and 3.5 cm sources. Temporal changes in source structure
could also be studied by having EIT obtain partial field-of-view images
(!9') at 171, 195, 284 and 304 Angstroms on timescales of a few minutes
or less. Exxplosive events lasting only a few tens of seconds will be
identified from their broadened line profiles seen in the
one-dimensional SUMER and CDS slits. Finally, the VLA and SOHO data
will be compared with Yohkoh soft X-ray images in order to study
changes in source structure or plasma parameters that might accompany
explosive events, energetic jets or flaring bright points.
Last June 14, 15 and 16, CDS cycled through seqnences of the
following lines:
Si XII 520 A, Fe XIV 334 A, Ca X, 557 A, Si X 347 A,
He I, 584 A, Fe XIV 353 A, Si X 356 A, O III 599 A,
Fe XVI 360 A, Fe XII 364 A, Mg IX 368 A and OV 629 A.
SUMER, at the same time, went through something like the following
sequence:
O IV 1407.38, 1404.81 A
OV 629.73 A
MG X 624.940
Fe XII 1242.01
N V 1242.80
Ne VIII 770.409
780.324
C IV 1548.20
C IV 1550.77
EIT, in additionto taking its normal sequence of full-disk images
at 171, 195, 284 and 304 A, also took partial-frame (9') images
of a region near the west limb at these wavelengths.
Basically, if CDS could image a couple of lines with formation
temperatures of ~5 x 10**5 K and 1.5 x 10**6 K,as often as
possible over a field of view of 4' x 4', that would be very useful.